I’m aware that my option may seem more esoteric than others, but I prefer to make these types of diagrams using TikZ. You can knock out a bunch of text-filled rounded rectangles with connecting arrows in a heartbeat. Obviously the downside is the learning curve, but the upside is the versatility—I create all of my (non-data-plotting) figures and diagrams like this—there’s very little you can’t draw if you’re willing to learn the language.
 One additional comment I would make is that diagramming in Latex/Tikz gives you the ability to version control the entire process, trying out various branches of the diagram in a lossless way. I find this invaluable.